Fully remove pytest command

pytest-runner has been removed, and it explicitly explains that tests_require
is deprecated in setuptools. There’s no need to keep it.

Related to #367.

setup.cfg

@@ -48,8 +48,6 @@ install_requires =
   defusedxml
   pillow
   tinycss2
-tests_require =
-  pytest
 python_requires = >= 3.5
 
 [options.entry_points]
@@ -71,9 +69,6 @@ test =
 [bdist_wheel]
 python-tag = py3
 
-[aliases]
-test = pytest
-
 [tool:pytest]
 norecursedirs =
   build